With the use of gene transfer technologies, T cells can be genetically modified to stably express antibody binding domains on their surface that endow the T cells with specificities that are independent of the constraints imposed by the major histocompatibility complex (MHC). Chimeric antigen receptors (CARs) represent synthetic proteins expressed on T-cells (CART cells) that fuse an antigen recognition fragment of an antibody (e.g., an scFv, or single-chain variable region fragment) with an intracellular domain of the CD3-zeta chain. Upon interaction with a target cell expressing the scFv's cognate antigen, CARs expressed on T cell cells can trigger T-cell activation leading to target cell killing (also referred to as target cell lysis). When combined with additional costimulatory signals such as the intracellular domain of CD137 or CD28, these receptors are also capable of generating proliferation. However, some of this proliferation appears to be antigen-independent, unlike normal T cell receptor (TCR) responses (Milone et al., 2009, Mol Ther 17(8):1453-64). Artificial receptors do not fully reproduce the intracellular signal transduction produced by natural TCR binding to antigenic peptide complexed with MHC molecules (Brocker, 2000, Blood 96(5):1999-2001). The signaling defects may limit the long-term survival of CART cells upon adoptive transfer in the absence of high levels of cytokines like IL-2 (Lo et al., 2010, Clin Cancer Res 16(10):2769-80). They also have altered regulation that might be beneficial in some anti-cancer applications (Loskog et al., 2006, Leukemia 20(10):1819-28), but these regulatory defects also lead to potential challenges to controlling their “off-target” activity against normal tissues that also express antigen, even at extremely low levels. These “off-target” effects are a serious limitation to CAR-based therapeutics, and have resulted in probable deaths during early Phase I evaluation of CAR-modified T cells (Morgan et al., 2010, Mol Ther 18(4):843-51).
Thus, there is a need in the art for alternative approaches for constructing CARs that overcome the limitations to current CAR-based therapeutics. The present invention addresses this unmet need in the art.